1. Field of the Invention
This invention relates to the generation of high purity sulfur dioxide by submerged combustion in liquid sulfur. The invention especially relates to the integration of a recycle of sulfur maintained at relatively high temperature to eliminate plugging and other control problems associated with unusual viscosity characteristics of elemental sulfur such as temperature cycling of liquid sulfur in the reactor.
2. Description of the Prior Art
High purity sulfur dioxide can be generated by the Calabrian process, as described in U.S. patent application, Ser. No. 07/376,836, "PROCESS FOR HIGH PURITY SULFUR DIOXIDE PRODUCTION", Davis et al., filed Jul. 7, 1989, and other processes employing submerged combustion as described subsequently herein. In these processes, the sulfur dioxide is formed when an oxygen-containing gas is injected beneath the surface of a pool of molten sulfur maintained above its auto-ignition temperature. Before travelling upward and reaching the surface of the liquid sulfur, oxygen in the oxygen-containing gas is essentially completely consumed in the combustion reaction of sulfur to sulfur dioxide. One distinct advantage of submerged combustion is the production of a sulfur dioxide stream essentially free of sulfur trioxide When oxygen is used as the oxygen-bearing gas, recovery and purification of the sulfur dioxide is further simplified.
For the processes using submerged combustion to make sulfur dioxide, the heat of combustion from the sulfur dioxide reaction heats both the liquid sulfur and the gas passing through it. Because the liquid sulfur is maintained at or above its auto-ignition temperature and is heated continuously by the combustion reaction, substantial amounts of sulfur are vaporized into the sulfur dioxide containing gas leaving the liquid sulfur. This effect was recognized in U.S. Pat. No. 2,595,447 to take advantage of a subsequent combustion of the vaporized sulfur.
When sulfur dioxide is desired as the product of submerged combustion, it is necessary to remove the sulfur vaporized into the sulfur dioxide containing gas to a level required by the downstream process user. The vaporized sulfur is most effectively removed from the sulfur dioxide containing gas by indirect heat transfer causing condensation of the sulfur but not of the sulfur dioxide. Because the sulfur dioxide containing gas is typically cooled to 300.degree. F. or less, condensing sulfur passes through a high viscosity transition in the temperature range of 310.degree. to 600.degree. F. The viscosity of liquid sulfur is relatively low outside of this temperature range (less than 2000 cps), but within it the viscosity rises to approximately 92,000. Such a rise in viscosity is noted by Paskall in FIG. 5 of the article "Sulphur Condenser Function and Problem Areas" in the Western Research Sulphur Seminar (Amsterdam, 1981). High viscosity liquid sulfur accumulates in the condensers of submerged combustion processes and interferes with heat transfer and free flow of liquid sulfur from the condenser. Plugging and erratic behavior of such condensers is a major concern in applying the submerged combustion processes and has limited commercial application.
Compounding this problem of liquid sulfur viscosity in the condenser has been the increasing, advantageous use of pure oxygen as the oxygen-containing gas for submerged combustion. Pure oxygen is used as the oxygen-containing gas to improve recovery and purification of the sulfur dioxide from the vapor, since, as compared to the use of the less costly air, nitrogen has been removed as a diluent. While advantageous for final recovery and purification of sulfur dioxide, elimination of nitrogen from the oxygen-containing gas increases the amount of sulfur vaporized into the sulfur dioxide containing gas while at the same time reducing the amount of non-condensable vapor passing through the indirect heat transfer cooling step.
Thus, in using pure oxygen as the oxygen-containing gas, submerged combustion processes must condense relatively more vaporized sulfur from the sulfur dioxide containing gas, adding to the problem of liquid sulfur removal from the condenser. Also, less non-condensable vapor passes through the condenser in relation to the amount of sulfur condensed there, resulting in reduced vapor cooling requirements in the condenser. Whatever the benefit of reducing non-condensable vapor cooling in this cooling step, such benefit is negligible when compared to the problem of removing additional high viscosity liquid sulfur from the condenser.
Therefore, the heat of combustion from the sulfur dioxide reaction described in the prior art is typically absorbed by cooled liquid sulfur added to the combustion zone. That cooled liquid sulfur is heated by the combustion reaction and is withdrawn as hot liquid or vaporized sulfur to be cooled and returned to the combustion zone.
U.S. Pat. No. 2,726,933 directly quenches with liquid sulfur the gas leaving the surface of the liquid sulfur in the combustion zone to produce a vapor at 550.degree. to 650.degree. F. (substantially below the boiling point of sulfur for the reactor pressure). A second direct quench with liquid sulfur reduces the gas temperature to 250.degree. to 310.degree. F. The liquid sulfur heated by the direct quenches is withdrawn from the respective pools of liquid sulfur, cooled, and returned to the direct quench steps. The total sulfur recirculation rate to the direct quench is 25 to 60 times the sulfur consumption rate. Because U.S. Pat. No. 2,726,933 prefers that the majority of the heat of combustion is absorbed by the cooled liquid sulfur fed directly to the combustion zone, the combustion zone must operate as near the auto-ignition temperature as possible in order to accommodate the 550.degree. to 650.degree. F. temperature range of the vapor leaving the zone of the first quench. This presents operational problems if the temperature of the liquid sulfur drops below the auto-ignition temperature and the combustion reaction stops.
U.S. Pat. No. 4,046,867 is directed to the scrubbing of the sulfur dioxide containing gas generated by a submerged combustion process, with dilute ammonia to remove the last traces of elemental sulfur. The patent additionally discloses a vertical tube condenser situated above the pool of liquid sulfur where submerged combustion takes place. Vaporized sulfur condenses in the vertical tubes. Some or all of the condensed sulfur drains directly back to the pool of liquid sulfur.
The outlet temperature of the condenser in U.S. Pat. No. 4,046,867 is preferably below 310.degree. F., which is repeatedly emphasized as the highest condenser outlet temperature to be used. The vapor from the condenser outlet is further subjected to direct contact with slightly cooler liquid sulfur. This liquid sulfur contact step is not intended to perform significant cooling of the vapor, at the most cooling it from 310.degree. or 300+ F. to over 240.degree. F. In addition, the vertical tube condenser permits condensation of sulfur from the upwardly flowing, sulfur dioxide containing gas on the inside surface of vertical condenser tubes. Without substantial additional cooling of the liquid sulfur in the reactor zone and a subsequent reduction of vaporized sulfur entering the condenser, the quantity of sulfur condensed from the reactor effluent will necessarily tend to plug the condenser tubes due to the very high viscosity zone encountered. Indeed, the detailed description of the invention states that the "Temperature of the sulfur in sulfur dioxide generator (4) is between about 550.degree. F. and desirably below its boiling point of 832.degree. F." The detailed description further states "If desired, pool of molten sulfur (5) may be cooled, as by externally located heat exchange means (not shown)." To maintain the desirable temperature in the sulfur dioxide generator below the sulfur boiling point such heat exchange is required. Merely returning cooled, condensed sulfur to the reactor in the manner shown in the figures does not reduce sulfur dioxide generator temperature to below the boiling point of liquid sulfur for the conditions described in the patent.
The vertical tube sulfur condenser has been almost entirely eliminated from industrial applications, as noted by Paskall and Sames, "Sulphur Condenser Function and Problem Areas" (page 7), Western Research Publishing Company, 1990. One skilled in the art would thus not use the condenser described in U.S. Pat. No. 4,046,867. Nothing in that patent suggests what type of heat transfer device might replace its vertical tube condenser in sequence and function and still produce the anticipated operation.
The Calabrian process, among its other advantages, solves the problem of replacing the vertical tube condenser of U.S. Pat. No. 4,046,867. Vapor from the liquid sulfur pool where submerged combustion takes place is cooled in two steps. First, a cooling jacket applied to a portion of the vapor space of the reactor condenses some sulfur on the wall of the reactor. The condensed sulfur then drains back to the liquid sulfur pool in the reactor. The temperature of the vapor leaving the reactor is preferably 800.degree. F.
A subsequent condenser further cools the vapor leaving the reactor and allows the sulfur condensed there to flow to a sulfur storage tank. The outlet temperature of the inclined condenser is maintained preferably between 240.degree. to 260.degree. F. It is the partial condensation in the reactor and/or the unique configuration and orientation of the condenser which overcomes the viscosity problem described above.
In the sulfur storage tank, liquid sulfur from two sources mix. The two sources of liquid sulfur are (1) that condensed in the condenser and (2) liquid sulfur from a make-up supply. From this pool of mixed liquid sulfur in the sulfur storage tank, liquid sulfur flows by gravity to the reactor.